Dislocation Creep Processes Research Articles

High‐temperature deformation of dry diabase with application to tectonics on Venus

We have performed an experimental study to quantify the high‐temperature creep behavior of natural diabase rocks under dry deformation conditions. Samples of both Maryland diabase and Columbia diabase were investigated to measure the effects of temperature, oxygen fugacity, and plagioclase‐to‐pyroxene ratio on creep strength. Flow laws determined for creep of these diabases were characterized by an activation energy of Q = 485±30 kJ/mol and a stress exponent of n = 4.7±0.6, indicative of deformation dominated by dislocation creep processes. Although n and Q are the same for the two rocks within experimental error, the Maryland diabase, which has the lower plagioclase content, is significantly stronger than the Columbia diabase. Thus the modal abundance of the various minerals plays an important role in defining rock strength. Within the sample‐to‐sample variation, no clear influence of oxygen fugacity on creep strength could be discerned for either rock. The dry creep strengths of both rocks are significantly greater than values previously measured on diabase under “as‐received” or wet conditions [Shelton and Tullis, 1981; Caristan, 1982]. Application of these results to the present conditions in the lithosphere on Venus predicts a high viscosity crust with strong dynamic coupling between mantle convection and crustal deformation, consistent with measurements of topography and gravity for that planet.

High strain rate torsional behavior of an ultrahigh carbon steel (1.8 Pct C-1.6 Pct Al) at elevated temperature

The elevated temperature mechanical properties of a 1.8 pct C-1.6 pct Al ultrahigh carbon steel (UHCS-1.8C-1.6Al) is described in the temperature range from 750 °C to 1150 °C and in the strain rate range from 0.2 to 26 s−1. A torsion test apparatus was used which permitted rapid cooling (50 °C per second) immediately after fracture to establish microstructure-processing-property relations. The strength-strain rate relation of the UHCS-1.8C-1.6Al material correlates well with a lattice diffusion-controlled dislocation creep process. The present data, together with other high carbon steels data, predict that austenite containing a high amount of carbon in solution has a high stacking fault energy. The ductility of the UHCS-1.8C-1.6Al is maximum at 1050 °C. This indicates that successful deep die forging and other mechanical processing operations at high strain rates should be performed at this temperature. The microstructure of the deformed samples consisted of a matrix of pearlite with some undissolved spherical carbides when rapidly cooled from 900 °C and 1050 °C and of a thin network of proeutectoid carbides when cooled from 1150 °C. High hardnesses in the range of Rockwell C 42 to 50 are obtained for such structures.

Quartz deformation in a very fine grained quartzo-feldspathic mylonite: a lack of evidence for dominant grain boundary sliding deformation

Microstructural and microtextural investigations by transmission electron microscopy (TEM) have been performed on very fine grained quartzo-feldspathic mylonites from the Red Bank Deformed Zone, Central Australia. The study of the quartz reveals a mean grain size (2.3 ± 0.8μm) and microstructural features indicative of dislocation creep processes within the very fine grained quartz. Analysis of local crystallographic preferred orientation (CPO) by TEM, showed a strong CPO for both the c- and a-axes. The crystallographic relationships between adjacent grains and subgrains are consistent with dominant 〈 c + a〉 and 〈 a〉 slip. Microstructural and microtextural studies also show that the recrystallization mechanism included local grain boundary migration and the formation of high angle grain boundaries from low angle (sub)grain boundaries by progressive misorientation. We found no abnormal grain rotations as would be expected to result from grain boundary sliding processes. This suggests that grain boundary sliding was not significant and the dominant deformation mechanism was dislocation creep.

The effect of temperature on the fracture mechanism in 2014A1/15vol.%Al 2O 3 composite

The tensile fracture strain, stress and fracture mode for a discontinuously reinforced aluminum matrix composite, 2014Al/15vol.%Al 2O 3, were determined and compared with those of the unreinforced matrix material, 2014A1, at various temperatures. Tests were conducted under uniaxial tension at elevated temperatures with a strain rate of 0.1 s −. It was found that the tensile fracture strain as well as fracture stress of the composite were lower than those of the matrix material. The tensile fracture mode changed from transgranular fracture to intergranular fracture between 400 °C and 500 °C for both materials. For the composite, at temperatures below 400 °C the growth and coalescence of voids occurred via a dislocation creep process primarily along the AlAl 2O 3 interface. Above 400 °C voids initiated and grew at the AlAl 2O 3 interface and grain boundaries via a diffusion creep process. The void growth was found not along the tensile direction but along the AlAl 2O 3 interface and grain boundaries, and this resulted in a low fracture strain. A method for determining quantitatively the characteristics of the void initiation and growth is discussed.

Mechanical behavior of fine-grained Mg-6.5Li at elevated temperature

A Mg-6.5 wt. % Li alloy containing 80% hep alpha phase and 20% bcc beta phase was processed to achieve an average grain size of 5.9 μm. Strain-rate-change tests were performed in the temperature range from 398 K to 573 K. Two types of creep behavior were observed. A stress exponent of five, obtained at low temperatures and high stresses, is attributed to a diffusion-controlled dislocation creep process in the alpha matrix. A stress exponent of three, obtained at high temperatures and low stresses, is attributed to a solute-drag controlled dislocation creep process in the alpha matrix.

Mechanical behavior and interface design of MoSi 2-based alloys and composites

The mechanical behavior of hot pressed MoSi 2-based composites containing Mo 5Si 3, SiO 2, CaO and TiC as reinforcing second phases was investigated in the temperature regime 1000–1300 °C. The effects of strain rate on the flow stress for Mo 5Si 3 − , SiO 2 − and CaO-containing composites are presented. Effects of several processing routes and microstructural modifications on the mechanical behavior of MoSi 2Mo 5Si 3 composites are given. Of these four composite additions, Mo 5Si 3 and CaO produce strengthening of MoSi 2 in the temperature range investigated. SiO 2 greatly reduces the strength, consistent with the formation of a glassy phase at interface and interphase boundaries. TiC reduces the flow stress of MoSi 2 in a manner that suggests dislocation pumping into the MoSi 2 matrix. The strain rate effects indicate that dislocation creep (glide and climb) processes operate over the temperature range investigated, with some contribution from diffusional processes at the higher temperatures and lower strain rates. Erbium is found to be very effective in refining the microstructures and in increasing the hardness and fracture properties of MoSi 2Mo 5Si 3 eutectics prepared by arc melting. Initial results on microstructural modeling of the deformation and fracture of MoSi 2-based composites are also reported.

State of strain in mylonites from the western Blue Ridge province, southern Appalachians: the role of volume loss

This study examines the state of finite strain in quartzo-feldspathic mylonites from two localities in the western Blue Ridge province of the southern Appalachians. Mineral shape fabric and whole-rock chemical and modal data suggest that the mylonites deformed by inhomogeneous shortening normal to the foliation and progressive shear parallel to the foliation. The deformation involved substantial bulk-rock volume loss. Strain was partitioned between solution transfer processes in feldspar and dislocation creep processes in quartz and micas at temperatures of between 300 and 450°C. Initially feldspars underwent grain-size reduction by cataclasis but at advanced stages of mylonitization alkali feldspar grains display subrounded oblate shapes (0 < k < 0.5) which are attributed to dissolution processes normal to the foliation. Quartz deformed by crystal-plastic processes and quartz grains and ribbons display a range of k values (0.2 < k < 0.9) reflecting heterogeneous deformation. Whole-rock mylonite samples are enriched by a factor of 3 relative to their gneissic protolith in ‘immobile’ trace elements such as Ti, Zr, Y, P and V, and the REE which is attributed to the concentration of residual phases such as zircon, apatite, epidote and ilmenite observed in the mylonites. The trace element enrichments are interpreted as due to large (>60%) bulk volume loss. Density changes were minor. On the basis of feldspar grain shapes and modal and whole-rock chemical data, volume loss was largely accommodated by fluid infiltration and incongruent dissolution of alkali feldspar (producing muscovite) and involved the loss of alkalis and silica to the fluid phase. The mylonites may have nucleated as solution zones which subsequently underwent strain softening and displacement parallel to the zone boundaries.

Harper-dorn creep in pure metals

Evidence is presented that Harper-Dora (H-D) viscous creep is a diffusion-controlled, dislocation creep process. The factors influencing H-D creep are the same as those influencing power-law creep, namely dislocation density, subgrain or grain size, stacking fault energy, and elastic modulus. H-D creep has been observed, and is analyzed, for seven different metals: Al, Pb, Sn, α-Zr, α-Ti, α-Fe, and β-Co. In addition, seven metals (α-Fe, Ni, Ag, β-Co, Cu, Mo, and Cr) which were originally believed to be controlled by diffusional (Nabarro-Herring) creep at low stresses, are shown to be controlled by Harper-Dorn creep. An internal stress assisted creep model is presented which correlates data in both the power-law and H-D creep regimes. The model predicts, quantitatively, the conditions under which H-D creep will dominate plastic flow.

The activation energies for plastic flow in a superplastic copper alloy

A superplastic copper alloy, which is nominally single phase, exhibits a sigmoidal relationship between strain rate and stress dividing the behavior into three regions. Only a very small amount of primary creep occurs in the low stress region I and the intermediate stress region II, and temperature cycling experiments show that the activation energies are independent of the total strain within the extended steady-state region. The activation energies are ∼ 179 kJ mol −1 in region I and ∼ 144 kJ mol −1 in region II, and these values are similar to those anticipated for lattice and grain boundary diffusion, respectively. The activation energy in the high stress region III is ∼ 159 kJ mol −1, suggesting that grain boundary sliding occurs to an appreciable extent in this region in addition to an intragranular dislocation creep process.

Theoretical equation for steady-state dislocation creep in a material having a threshold stress

A model of the dislocation creep process has been developed and shown to give reasonable predictions for the thermal and in-reactor creep rates of some pure crystalline materials. Its application to solid solutions has also been described and in the present paper the model is further developed for materials (such as those hardened by a dispersion) in which, arguably, dislocation creep only occurs when a threshold stress is exceeded. The predictions of the resultant creep equation are compared with the results of experiment and reasonable agreement is found. In particular the absolute rate of creep for a 'SAP’ type material is forecast with reasonable accuracy while the dependence of the stress and temperature sensitivity (of the creep rate) upon the absolute magnitudes of stress and temperature are correctly mirrored by the theoretical equation.